Just as the title says, I'm needing something to drive my DT 880 600 Ohms. Would an STX essence be adequate or what amp should I get? I have a xonar d2x at the moment

Technically, the amplifier of the STX is sufficient in terms of power and measured sound quality. So it is a decent option for a reasonable price. If you can get a pre-built O2 amplifier instead, it may be an improvement for a similar price (the D2X is good as a DAC). Tube amplifiers are often recommended for the DT880 for a "warmer" (although from a purely technical point of view not better) sound, but the good ones are obviously not cheap.

I've had a look around some other similar threads and saw a few recommendations for the FiiO E9 so I bought that. I hear people saying it's pointless to put the E9 and an STX together, so the d2x should be sufficient for now. Would there be a point going for an STX in the future?

As an amplifier, the ST(X) performs similarly to the E9, it is also based on the same chip. Perhaps you would get some improvement in distortion and noise, partly due to the better DAC of the ST/STX cards. I do not know how much, if any audible difference there would be in practice, though. But if you were thinking about getting the ST or STX, then it would have been best to buy that in the first place, and sell the D2X if you do not need it. Of course, the E9 is useful as a stand-alone desktop amplifier that you can use from other sources without the PC.

I've had a look around some other similar threads and saw a few recommendations for the FiiO E9 so I bought that. I hear people saying it's pointless to put the E9 and an STX together, so the d2x should be sufficient for now. Would there be a point going for an STX in the future?

It looks like he already bought something.

how do you find the E9 performance with your beyers?

And, i think its better going for an amp dac in the future...unless space is an issue and you want a soundcard.

I got the FiiO E9 in the mail and tested it out. It's actually faulty - the power LED is meant to come on when turning the knob but it doesn't happen at all. Amazon is sending me another one to be here tomorrow. I'll let you guys know what happens.